fugue, fuga, fuge, fuguette—a contrapuntal procedure (form) involving a musical subject (theme) that is successively imitated by multiple overlapping voices. There is a vast canon of rules (concepts) for creating elaborate and diverse fugues with an infinite variety of character based on fairly specific parameters. Fugues (or partial fugato) are found throughout classical music from the late Renaissance to the present.

quartet, quatuor [F], quartett [G], quartetto [I]—ensemble or work for four players the most important examples being the string quartet and the piano quartet

string quartet, Streichquartett [G], quatuor à cordes [F], quartetto d'archi [I], quartetto di cordi [I]—an ensemble as well as music written for that ensemble comprising 2 violins, viola and cello. This is one of the essential genres / forms / ensembles of chamber music and arguably a critical "core" of "classical" music. Explore the string quartet.

tema con variazioni, con variazioni, Tema e variazioni, theme and variations, Thema mit Variationen [G], Thème varié [F]—a common movement form featuring an initial theme followed by a series of variations on that theme, each of which adjusts any number of parameters to achieve variety, e.g. tempo, rhythm, key, instrumentation, etc.A challenge to the composers ingenuity and a delight for the listener